Water quality degradation is a problem of growing proportions. One measure of water quality is the amount of gross pollution that is carried from our roads and streets into our storm water system and eventually into the reservoirs that we rely on for potable water, our rivers and ultimately the ocean.
Gross pollution in the main (90%) comprises organic matter like branches, twigs, leaves, and soil, the remainder being nonorganic matter like plastic containers, bags, wrappers; paper; cans; and cigarette butts.
In metropolitan and township areas storm water catchment consists of road surfaces and the entry points for storm water into the underground portion of a storm water drainage system consists of one or more gully traps located along the sides of the road surfaces. There exist different types of gullies, the side entry and the surface grate being the two most common.
Serious academic and practical studies of road surface, curb and channel design relating to road usage and safety are combined with gully trap configuration (eg depression, grate, deflector, etc) studies to provide the optimum means by which to trap storm water, and the inevitable gross pollution carried by it, and directed into the drains below the road surface.
Such traps are designed to operate effectively in a range of storm water run off conditions including low flow to very high flow which ideally captures 100% of the flow but otherwise minimises the bypass of water and pollutants downstream of the trap.
Regardless though of the effectiveness of the traps, the problem identified by the inventors was how to effectively filter the full range of flows of water and prevent the entry of gross pollution into the underground storm water drainage systems without affecting the efficiency of the traps.
One approach to the solution of this problem is disclosed in U.S. Pat. No. 5,232,587 which describes the use of a two stage grate filter arrangement which in theory is meant to provide a path of least resistance to the storm water just internal and slightly below the entrance to the trap by providing a multi aperture grate and beyond that a further grate with larger apertures located rearward and internal of the trap opening.
As gross pollution of a size unable to pass through the grates builds up, the resistance to flow through the grates increases and it is soon found that the efficiency of the trap diminishes as is evidenced by the increasing amount of bypass flow. Eventually, the grates become covered with gross pollution and the trap is no longer useful. The volume of gross pollutants needed to produce this effect is quite small since the grates are located just below the road surface level which therefore necessitates frequent cleaning to maintain trap efficiency.
Thus the invention is directed to overcoming the problems described above while also reducing the maintenance commitment.